The demand for small, light weight consumer products has increased substantially in recent years. This demand for smaller, lighter weight products has forced manufacturers of these products to employ ultrathin, ultralight materials in, for example, the plastic housing of those products. These ultrathin plastic housings often have thicknesses on the order of between 0.20 and 0.50 millimeters.
These thin plastic housings are low weight, and often rely upon the structure of the item being housed for structural integrity and/or stability. However, they pose several challenges in the manufacturing arena, and particularly, in joining two or more such plastic pieces. Typically, ultrathin plastic parts must be bonded together with adhesives because standard ultrasonic welding techniques are not feasible.
Traditionally, thick plastic parts have been joined ultrasonically by aligning the parts, and initiating the weld along an energy director formed into one of the parts. The nature of the bond between the parts so joined is tensile, that is, resists the tendency of forces to tear it apart. Manufacturers have typically strengthened these types of ultrasonic welds by either increasing the weld depth, or increasing the size of the energy director, thus providing a larger weld area. However, adopting this approach with ultrathin plastic components results in numerous problems. For example, increased weld depth and/or energy director size also increases the likelihood of burning through one or more of the plastic parts, thus rendering the part unusable. Deep welds also cause "flash" or seepage of the weld from between the welded components. Welds which completely melt one or both of the parts degrade the plastic, causing concentrated stresses of the weld joint. These stresses result in weaker bonds, and also in a tendency for parts so joined to come apart upon, for example, impact.
Another problem associated with ultrasonically joining plastic parts arises from the tendency of plastic parts to become mis-aligned during the welding process. More particularly, the ultrasonic welding process is often a substantially manual procedure, whereby two or more plastic parts are brought into contact with one another and must be held so, as by the application of pressure to the two parts, until the weld is completed. The result is mis-formed parts which are at least cosmetically unacceptable, and at worst lacking in structural integrity.
Accordingly, there exists a need for a method of ultrasonically welding the parts, particularly ultrathin plastic parts, which provides for enhanced weld strength, and improved part alignment.